Capillary electrophoresis (CE) and other analytical techniques have greatly benefited from micromachining. Microfabrication technology provides a means for implementation of complex CE devices with a high degree of functionality (Woolley, A. T. and R. A. Mathies, “Ultra-high speed fragment separations using microfabricated capillary array electrophoresis chips”, Proc. Natl. Acad. Sci., USA, 91:11348–11352, 1994; Bousse, L. et al., “High performance DNA separations in microchip electrophoresis systems”, Proc. micro-TAS '98 Workshop, pp. 271–275, 1998) yielding fast and highly efficient separations. The majority of micromachined CE devices, however, have been fabricated using bonded, glass substrate technology with off-chip laser induced fluorescence detection. The advantages of miniaturization are greatly reduced when large detection systems are used in microanalysis.
A prior art method of on chip detection utilized a photodiode on silicon (Webster, J. R., et al., “Electrophoresis system with integrated on-chip fluorescence detection”, IEEE MEMS 2000 conference, Miyazaki, Japan, pp. 381–385, January 2000). However, compatibility of the silicon photodiode with plastic substrates, which are favored because of bio-compatibility and cost, is difficult to achieve. Additionally, the optical detection method required additional optical devices such as LEDs, lasers and optical filters for efficient detection thereby increasing the systems bulk.
Electrochemical detection methods have also been tried in micromachined CE, however, the methods tried thus far have been end column methods. End column detection limits resolution (Voegel, P. D. and R. P. Baldwin, “Electrochemical detection in capillary electrophoresis”, Electrophoresis, 18:2267–2278, 1997; Gavin, P. F. and A. G. Ewing, “Continuous separations with microfabricated electrophoresis-electrochemical array detection”, J. Am. Chem. Soc., 118:8932–8936, 1996). Inline electrochemical methods have been used in large scale HPLC, for example (Kafil, J. B., et al., “Quantitation of nucleic acids at the picogram level using HPLC with electrochemical detection”, Anal. Chem, 58:285–289, 1986). However, inline electrochemical detection methods have not been employed in micromachined CE because of problems in coupling detection and separation fields and in bubble generation in aqueous solution at the high CE fields.
What is needed is an inexpensive, easy to fabricate electrochemical detection system that provides improved resolution and a higher limit of detection.